1. Field of the Invention
The present invention generally relates to dual mode transmitters. More specifically, the present invention provides a dual mode transmitter having an adjustable power amplifier to provide both linear and non-linear amplification.
2. Background Art
A dual mode transmitter is capable of implementing linear modulation schemes and non-linear modulation schemes and transmitting signals generated therefrom. A linear modulation scheme typically produces a variable envelope signal. A non-linear modulation scheme typically produces a constant envelope signal. A variable envelope signal requires linear amplification to prevent distortion of information encoded in the amplitude of the variable envelope signal. A constant envelope signal does not require linear amplification since the amplitude of a constant envelope signal does not contain information. Constant envelope signals can therefore tolerate distortion from non-linear amplification provided the zero-crossings of the constant envelope signal are preserved.
Dual mode transmitters typically use a single power amplifier to amplify modulated signals prior to transmission. A power amplifier requires a relatively high bias voltage to provide linear amplification. A power amplifier, however, does not require a relatively high bias voltage to provide non-linear amplification. Therefore, using a linear power amplifier to amplify a constant envelope signal decreases efficiency since unneeded power is consumed. Alternatively, using a non-linear power amplifier to amplify a variable envelope signal causes intolerable distortion. Further, using multiple power amplifiers to accommodate different modes of modulation wastes space and increases operating costs and integration costs when the dual mode transmitter is implemented on a single semiconductor chip.